Turbochargers have long been known as a means for increasing both power and efficiency in internal combustion engines. In general, a turbocharger takes advantage of otherwise wasted energy from exhaust gas flowing out of the engine cylinders. In a typical design, exhaust gas is directed past a turbine coupled with a shaft. The exhaust gas induces a rotation of the turbine, thereby operating a compressor coupled to an end of the shaft opposite the turbine. The compressor in turn compresses air supplied to the engine cylinders. Among other things, increasing the pressure of air incident to the engine increases the oxygen supply, and hence the quantity of fuel that can be burned during a given cylinder combustion cycle. Thus, a conventional turbocharger not only has the potential to increase the power output of the engine, it takes advantage of energy in the form of exhaust gas pressure that may otherwise be lost.
A typical turbocharged engine may utilize one or more turbochargers. In multiple turbocharger designs, the devices may be operable independently of one another, in parallel, or serially such that air supplied to the engine undergoes plural pressurizing stages. “Compound” turbochargers are one known turbocharger design wherein a first shaft is coupled with a first turbine and first compressor, and a second shaft, coaxial with and extending within the first shaft, is coupled with a second turbine and second compressor. The turbine stages are arranged such that exhaust gas rotates the first turbine, then flows past and rotates the second turbine. Air initially pressurized by a “low pressure” compressor subsequently flows to a “high pressure” compressor, undergoing further compression before being supplied to the engine cylinders. The compound design can thus offer a dual-stage compression of the incident air.
A design challenge common to many turbochargers relates to the tendency for various of the turbocharger components to vibrate undesirably during operation. For example, non-axial vibrations in a turbocharger shaft can disrupt operation and efficiency or even result in shaft failure. Due in part to the relatively high rotational speeds of many turbocharger shafts, even those shafts having considerable bulk and stiffness can experience problematic vibrations.
Engineers have discovered over the years that one of the most problematic “types” of vibrations in a turbocharger shaft are those at frequencies equal to the natural frequencies of vibration of the shaft, or resonance frequencies. Every solid object has natural resonance frequencies and, when sufficient kinetic energy is imparted at a resonance frequency thereof, will vibrate at steadily increasing amplitudes so long as sufficient kinetic energy continues to be applied at that frequency. “Galloping Girdie” is a familiar example of a resonance vibration, in particular a torsional vibration, or twisting back and forth. Any integer multiple of a first natural resonance frequency can similarly induce resonance vibration. However, the first or lowest resonance frequency of a turbocharger shaft, known in the art as the “first bending critical,” is generally the most difficult to manage.
Many turbocharger shafts experience the first bending critical during operation within a speed range commonly desirable for turbochargers. In other words, a typical range of desired shaft speeds happens to encompass the speed at which many turbocharger shafts resonate. In certain designs, it may be possible to accelerate the shaft rotation through the first bending critical fairly quickly, allowing the problem to be ignored. This approach is commonly taken with engines having a limited speed range.
In some operating schemes, however, the above approach works less well, if at all. Resonance vibration problems are particularly acute in systems operating in cooperation with an engine designed to operate over a relatively broad speed range, requiring regular increases or decreases in shaft speed, potentially passing through the first bending critical frequently. Engineers have also attempted to manage vibrational behavior of rotating shafts through various other means. The mass properties, shaft stiffness and shaft geometry can all be varied to alter the resonance frequencies of the shaft, as well as its capacity for handling the same. Nevertheless, there is always room for improvement, particularly where other design requirements of the shaft assembly and associated engine limit the extent to which the shaft itself can be changed. Where certain compound turbochargers are concerned, the nature of coaxial shafts can introduce still further challenges.
One example of a compound turbocharger is known from U.S. Pat. No. 4,155,684 to Curiel et al. Curiel et al. describe a two-stage exhaust-gas turbocharger having coaxial inner and outer turbocharger shafts.
Each of the shafts of Curiel et al. are rotatably supported in bearings in the housing, disposed outside of the turbocharger wheels. While Curiel et al. represents one relatively compact compound turbocharger design, the system is not without shortcomings, particularly with regard to its capacity for managing resonance vibrations of the inner turbine shaft.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.